Light-emitting diodes (LEDs) are a promising technology for flat-panel displays and area illumination lamps. The technology relies upon thin-film layers of organic materials coated upon a substrate. LED devices generally can have two formats known as small molecule devices such as disclosed in U.S. Pat. No. 4,476,292, issued Oct. 9, 1984 to Ham et al., and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190, issued Sep. 21, 1993 to Friend et al. Either type of OLED device may include, in sequence, an anode, an organic EL element, and a cathode. The organic electroluminescent (EL) element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EML) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EL layer. Tang et al. (Applied Physics Letter, 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous LEDs with alternative layer structures, including organic or polymeric materials, or inorganic materials, have been disclosed and device performance has improved.
Light is generated in an LED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron transport layer (ETL) and the hole transport layer (HTL) and recombine in the emissive layer. Many factors determine the efficiency of this light generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of EML can determine how efficiently the electrons and holes be recombined and result in the emission of light, etc.
LED devices can employ a variety of light-emitting organic materials patterned over a substrate that emit light of a variety of different frequencies, for example, red, green, and blue, to create a full-color display. However, patterned deposition is difficult, requiring, for example, expensive metal masks. Alternatively, it is known to employ a combination of emitters, or an unpatterned broad-band emitter to emit white light together with patterned color filters, for example, red, green, and blue, to create a full-color display. The color filters may be located on the substrate, for a bottom-emitter, or on the cover, for a top-emitter. For example, U.S. Pat. No. 6,392,340, issued May 21, 2002 to Yoneda et al., illustrates such a device. However, such designs are relatively inefficient, since approximately two thirds of the light emitted may be absorbed by the color filters.
It has been found that one of the key factors that limits the efficiency of LED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the LED devices. Due to the relatively high optical indices of the organic and transparent electrode materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the LED devices and make no contribution to the light output from these devices. Because light is emitted in all directions from the internal layers of the LED, some of the light emits directly from the device, and some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost in this manner.
A typical LED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic or inorganic layers, and a reflective cathode layer. Light generated from such a device may be emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent cathode layer. Light generated from such an alternative device may be emitted through the top transparent electrode. This is commonly referred to as a top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions.
In any of these LED structures, the problem of trapped light remains. Referring to FIG. 7, a bottom-emitting LED device as known in the prior art is illustrated having a substrate 10 (in this case transparent), a transparent first electrode 12, one or more layers of light-emitting material 14, a reflective second electrode 16, a gap 19 and a cover 20. First electrode 12, the one or more layers of light-emitting material 14 and reflective second electrode 16 form a light-emitting element 8. The gap 19 is typically filled with desiccating material. Light emitted from one of the material layers 14 can be emitted directly out of the device, through the transparent substrate 10, as illustrated with light ray 1. Light may also be emitted and internally guided in the transparent substrate 10 and material layers 14, as illustrated with light ray 2. Additionally, light may be emitted and internally guided in the layers of material 14, as illustrated with light ray 3. Light rays 4 emitted toward the reflective electrode 16 are reflected by the reflective first electrode 12 toward the substrate 10 and follow one of the light ray paths 1, 2, or 3. In some prior-art embodiments, the electrode 16 may be opaque and/or light absorbing. This LED display embodiment has been commercialized, for example, in the Eastman Kodak LS633 digital camera. The bottom-emitter embodiment shown may also be implemented in a top-emitter configuration with a transparent cover and top electrode.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. For example, diffraction gratings have been proposed to control the attributes of light emission from thin polymer films by inducing Bragg scattering of light that is guided laterally through the Emissive Layers; See “Modification Of Polymer Light Emission By Lateral Microstructure” by Safonov et al., Synthetic Metals 116, 2001, pp. 145-148, and “Bragg Scattering From Periodically Microstructured Light Emitting Diodes” by Lupton et al., Applied Physics Letters, Vol. 77, No. 21, Nov. 20, 2000, pp. 3340-3342. Brightness enhancement films having diffractive properties and surface and volume diffusers are described in WO2002/037568 entitled, “Brightness and Contrast Enhancement of Direct View Emissive Displays” by Chou et al., published May 10, 2002. The use of micro-cavity techniques is also known; for example, see “Sharply Directed Emission In Organic Electroluminescent Diodes With An Optical-Microcavity Structure” by Tsutsui et al., Applied Physics Letters 65, No. 15, Oct. 10, 1994, pp. 1868-1870. However, none of these approaches cause all, or nearly all, of the light produced to be emitted from the device.
Chou, in WO2002/037580 and Liu et al. in U.S. Patent Publication 2001/0026124, taught the use of a volume or surface scattering layer to improve light extraction. The scattering layer is applied next to the organic layers or on the outside surface of the glass substrate and has an optical index that matches these layers. Light emitted from the OLED device at higher than a critical angle that would have otherwise been trapped can penetrate into the scattering layer and be scattered out of the device. The efficiency of the OLED device is thereby improved, but still has deficiencies as explained below.
U.S. Pat. No. 6,787,796 entitled, “Organic Electroluminescent Display Device And Method Of Manufacturing The Same”, issued Sep. 7, 2004 to Do et al., describes an organic electroluminescent (EL) display device and a method of manufacturing the same. The organic EL device includes a substrate layer, a first electrode layer formed on the substrate layer, an organic layer formed on the first electrode layer, and a second electrode layer formed on the organic layer, wherein a light loss preventing layer having different refractive index areas is formed between layers of the organic EL device having a large difference in refractive index among the respective layers. U.S. Publication 2004/0217702 entitled, “Light Extracting Designs For Organic Light Emitting Diodes”, published Nov. 4, 2004 by Garner et al., similarly discloses use of microstructures to provide internal refractive index variations or internal or surface physical variations that function to perturb the propagation of internal waveguide modes within an LED. When employed in a top-emitter embodiment, the use of an index-matched polymer adjacent the encapsulating cover is disclosed.
Light-scattering layers used externally to an OLED device are described in U.S. Publication 2005/0018431 entitled, “Organic Electroluminescent Devices Having Improved Light Extraction”, published Jan. 27, 2005, by Shiang and U.S. Pat. No. 5,955,837, issued Sep. 21, 1999, by Horikx, et al. These disclosures describe and define properties of scattering layers located on a substrate in detail. Likewise, U.S. Pat. No. 6,777,871, issued Aug. 17, 2004, by Duggal et al., describes the use of an output coupler comprising a composite layer having specific refractive indices and scattering properties. While useful for extracting light, this approach will only extract light that propagates in the substrate (illustrated with light ray 2) and will not extract light that propagates through the organic layers and electrodes (illustrated with light ray 3).
However, scattering techniques, by themselves, may cause light to pass through the light-absorbing material layers multiple times where they are absorbed and converted to heat. Moreover, trapped light may propagate a considerable distance horizontally through the cover, substrate, or organic layers before being scattered out of the device, thereby reducing the sharpness of the device in pixellated applications such as displays. For example, a pixellated bottom-emitting LED device may include a plurality of independently controlled sub-pixels (as shown in FIG. 7) and a scattering layer (not shown in FIG. 7) located between the transparent first electrode 12 and the substrate 10. A light ray 2, 3, or 4 emitted from the light-emitting layer may be scattered multiple times by a scattering layer (not shown in FIG. 7), while traveling through the substrate 10, organic layer(s) 14, and transparent first electrode 12 before it is emitted from the device. When the light ray 2, 3, or 4 is finally emitted from the device, the light ray 2, 3, or 4 may have traveled a considerable distance through the various device layers from the original sub-pixel location where it originated to a remote sub-pixel where it is emitted, thus reducing sharpness. Most of the lateral travel occurs in the substrate 10, because that is by far the thickest layer in the package. Also, the amount of light emitted is reduced due to absorption of light in the various layers.
U.S. Patent Publication 2004/0061136 entitled, “Organic Light Emitting Device Having Enhanced Light Extraction Efficiency” by Tyan et al., describes an enhanced light extraction OLED device that includes a light scattering layer. In certain embodiments, a low index isolation layer (having an optical index substantially lower than that of the organic electroluminescent element) is employed adjacent to a reflective layer in combination with the light scattering layer to prevent low angle light from striking the reflective layer, and thereby minimize absorption losses due to multiple reflections from the reflective layer. The particular arrangements, however, may still result in reduced sharpness of the device.
Co-pending, commonly assigned US Publication 2006/0186802, published 24 Aug. 2006, by Cok et al., describes the use of a transparent low-index layer having a refractive index lower than the refractive index of the encapsulating cover or substrate through which light is emitted and lower than the organic layers to enhance the sharpness of an OLED device having a scattering element. US Publication 2005/0194896, published Sep. 8, 2005 by Sugita et al., describes a nano-structure layer for extracting radiated light from a light-emitting device together with a gap having a refractive index lower than an average refractive index of the emissive layer and nano-structure layer. Such disclosed designs, however, are difficult to manufacture, and may still not extract all of the available light in the presence of conventional encapsulation layers that may be employed to protect the OLED from environmental damage.
It is also well known that OLED materials are subject to degradation in the presence of environmental contaminants, in particular moisture. Organic light-emitting diode (OLED) display devices typically require humidity levels below about 1000 parts per million (ppm) to prevent premature degradation of device performance within a specified operating and/or storage life of the device. Control of humidity levels within a packaged device is typically achieved by encapsulating the device with an encapsulating layer and/or by sealing the device, and/or providing a desiccant within a cover. Desiccants such as, for example, metal oxides, alkaline earth metal oxides, sulfates, metal halides, and perchlorates are used to maintain the humidity level below the aforementioned level. See, for example, U.S. Pat. No. 6,226,890, issued May 8, 2001 to Boroson et al. describing desiccant materials for moisture-sensitive electronic devices. Such desiccating materials are typically located around the periphery of an OLED device or over the OLED device itself.
In alternative approaches, an OLED device is encapsulated using thin multi-layer coatings of moisture-resistant material. For example, layers of inorganic materials such as metals or metal oxides separated by layers of an organic polymer may be used as shown in prior-art FIG. 8. Referring to FIG. 8, a light-emitting element 8 is formed on a substrate 10. Alternating layers of organic material 40 and inorganic material 42 are formed over the light-emitting element 8 to protect the light-emitting element 8 from environmental contamination. Such coatings have been described in, for example, U.S. Pat. No. 6,268,695, issued Jul. 31, 2001 to Affinito, U.S. Pat. No. 6,413,645 issued Jul. 2, 2002 to Graff et al, and U.S. Pat. No. 6,522,067 issued Feb. 18, 2003 to Graff et al. A deposition apparatus is further described in WO2003/090260, entitled, “Apparatus for Depositing a Multilayer Coating on Discrete Sheets”, published Oct. 20, 2003, by Pagano et al. WO2001/082390 entitled “Thin-Film Encapsulation of Organic Light-Emitting Diode Devices”, published Nov. 1, 2001 by Ghosh et al., describes the use of first and second thin-film encapsulation layers made of different materials wherein one of the thin-film layers is deposited at 50 nm using atomic layer deposition (ALD) discussed below. According to this disclosure, a separate protective layer is also employed, e.g. parylene and/or SiO2. Such thin multi-layer coatings typically attempt to provide a moisture permeation rate of less than 5×10−6 gm/m2/day to adequately protect the OLED materials. In contrast, typically polymeric materials have a moisture permeation rate of approximately 0.1 gm/m2/day and cannot adequately protect the OLED materials without additional moisture blocking layers. With the addition of inorganic moisture blocking layers, 0.01 gm/m2/day may be achieved and it has been reported that the use of relatively thick polymer smoothing layers with inorganic layers may provide the needed protection. Thick inorganic layers, for example, 5 microns or more of ITO or ZnSe, applied by conventional deposition techniques such as sputtering or vacuum evaporation may also provide adequate protection, but thinner conventionally coated layers may only provide protection of 0.01 gm/m2/day. WO2004/105149 entitled, “Barrier Films for Plastic Substrates Fabricated by Atomic Layer Deposition” published Dec. 2, 2004 by Carcia et al., describes gas permeation barriers that can be deposited on plastic or glass substrates by atomic layer deposition (ALD). Atomic Layer Deposition is also known as Atomic Layer Epitaxy (ALE) or atomic layer CVD (ALCVD), and reference to ALD herein is intended to refer to all such equivalent processes. The use of the ALD coatings can reduce permeation by many orders of magnitude at thicknesses of tens of nanometers with low concentrations of coating defects. These thin coatings preserve the flexibility and transparency of the plastic substrate. Such articles are useful in container, electrical, and electronic applications. However, such protective layers also result in light trapped in the protective layers since they may be of lower index than the light-emitting organic layers.
There is a need therefore for an improved organic light-emitting diode device structure that avoids the problems noted above and improves the lifetime, efficiency, and sharpness of the LED device.